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This study evaluated the occurrence of extended-spectrum β-lactamases (ESBL) and associated resistance genes, integrons, and plasmid types as well as the genetic relatedness of enterobacterial isolates in the wastewater treatment plant (WWTP) of La Charguia, Tunis City (Tunisia). One-hundred water samples were collected at different points of the sewage treatment process during 2017–2019. Antimicrobial susceptibility was conducted by disc-diffusion method. blaCTX-M, blaTEM and blaSHV genes as well as those encoding non-β-lactam resistance, the plasmid types, occurrence of class1 integrons and phylogenetic groups of Escherichia coli isolates were determined by PCR/sequencing. Genomic relatedness was determined by MLST for selected isolates. Fifty-seven ESBL-producer isolates were recovered (47 E. coli, eight Klebsiella pneumoniae, one of the Citrobacter freundii complex and one of the Enterobacter cloacae complex). CTX-M-15 was the most frequently detected ESBL, followed by CTX-M-27, CTX-M-55 and SHV-12. One E. coli isolate harboured the mcr-1 gene. The following phylogroups/STs were identified among ESBL-producing E. coli isolates: B2/ST131 (subclade-C1), A/ST3221, A/ST8900, D/ST69, D/ST2142, D/ST38, B1/ST2460 and B1/ST6448. High number of isolates harboured the class 1 integrons with various gene cassette arrays as well as IncP-1 and IncFIB plasmids. Our findings confirm the importance of WWTPs as hotspot collectors of ESBL-producing Enterobacteriaceae with high likelihood spread to human and natural environment.

Extended-spectrum β-lactamase and their molecular mechanism in Enterobacteriaceae were analyzed in 126 fish samples of 9 various wild species, living in the lagoon of Bizerte in Tunisia. Fifty-nine (59) Gram-negative strains were isolated and identified as Escherichia coli (n=24), Klebsiella pneumonia (n=21), Citrobacter freundii (n=8), and Shigella boydii (n=6). Forty-seven ESBL producers were identified using the synergic test. β-Lactamase genes detected were blaCTX-M-1 (E. coli/15; K. pneumonia/8; C. freundii/1; Sh. boydii/1), blaCTX-M-1+ blaOXA-1 (E. coli/4; K. pneumonia/3), blaCTX-M-1+ blaTEM-1-a (K. pneumonia/2), blaCTX-M-15+ blaTEM-1-a (K. pneumonia/1; Sh. boydii/1), blaCTX-M-15+ blaOXA-1 (K. pneumonia/1), blaCTX-M-15 (E. coli/3; K. pneumonia/1; Sh. boydii/3), and blaCTX-M-9 (C. freundii/3). Most strains (84.7%) showed a multiresistant phenotype. qnrA and qnrB genes were identified in six E. coli and in ten E. coli+one K. pneumonia isolates, respectively. The resistance to tetracycline and sulfonamide was conferred by the tet and sul genes. Characterization of phylogenic groups in E. coli isolates revealed phylogroups D (n=20 strains), B2 (n=2), and A (n=2). The studied virulence factor showed prevalence of fimA genes in 9 E. coli isolates (37.5%). Similarly, no strain revealed the three other virulence factors tested (eae, aer, and cnf1). Our findings confirmed that the lagoons of Bizerte may be a reservoir of multidrug resistance/ESBL-producing Enterobacteriaceae. This could lead to indisputable impacts on human and animal health, through the food chain.

Significance and Impact of the Study: Increasing usage of antimicrobial compounds in human and veterinary medicine has contributed to the global emergence and transmission of antimicrobial-resistant bacteria, representing a major concern for human and animal health. According to many international and governmental organizations, the impact of antimicrobial resistance will be particularly significant in low-income regions across Africa, Asia, and Latin America. Accordingly, the development and implementation of multidisciplinary (i.e. One Health) evidence-based control programs and strategies are critical. Colistin is a 'last-resort' antimicrobial treatment for multidrug-resistant Gram-negative infections including pneumonia from Enterobacterales; however, high rates of colistin resistance have been reported globally, including Africa. A more profound understanding of the molecular mechanisms underlying colistin resistance is required for the development of effective surveillance programs and innovative therapies against multidrug-resistant bacteria.